The present invention relates generally to cameras and more specifically to an auxiliary illuminating device that has an adjustable color temperature.
When capturing an image with a digital camera, the source of illumination for the scene affects the colors captured by the camera. For indoor scenes the illumination source can vary widely and can include a tungsten bulb, a halogen lamp, a fluorescent lamp, sunlight coming in through a window, or even a xenon light. Each of these light sources has a different spectral energy distribution. The type of light source that creates light using a filament glowing at a high temperature (for example tungsten bulbs) are typically characterized by a color temperature defined as a Planckian radiator with a temperature 50 degrees higher than the filament of the light (see FIG. 1). The sun can also be characterized as a Planckian radiator but the loss of some wavelengths through scattering and absorption in the atmosphere causes significant differences from the Plankian radiator at those wavelengths. Because of the variation in the spectral power distribution of the sun, standard spectral power distribution curves have been developed. One of the standard curves is called D65 having a color temperature of 6500 k (see FIG. 2). Clouds in the sky can also affect the spectral distribution of energy reaching the scene from the sun. The time of day also affects the color temperature of the sun (noon vs. sunrise). The color temperature can be affected by whether the object is in direct sun light or in shadow.
The type of light source that excites a phosphor layer that then fluoresces (for example fluorescent lamps) tend to have spectral distributions that are unique to the phosphors in the lamp (see FIG. 3) in combination with the mercury vapor spectrum.
Each of these light sources has a different spectral power distribution that affects the colors captured in a scene by a camera. For example when you have a white object illuminated by a tungsten bulb the white object will appear yellow in the scene captured by the camera (assuming the camera is calibrated for normal daylight like D65). This is because the tungsten bulb does not produce much blue light. A white object is an object that reflects an equal amount of the red, green and blue light that hits the object. When a white object is illuminated by a tungsten bulb more red light is hitting the object than blue light and therefore more red light is reflected, causing the object to look yellow to the camera. The human eye adjusts to different illuminates and compensates for the color shift but a camera records the actual light in the scene. Hence, a camera without color compensation, or incorrect compensation, would generate images which would be perceived as being xe2x80x9coff colorxe2x80x9d.
Fortunately these color shifts caused by the illumination source can be corrected. This correction is typically called white balancing. There are many methods currently used to try to adjust the image to the proper white point (see U.S. Pat. No. 6,038,399).
One method looks for the brightest point in a scene and assumes that it should be white. The brightest point is adjusted until it is white and then this adjustment is used to balance the rest of the scene. This method operates on the assumption that the brightest point in a scene is from a white object or from a specular reflection, for example, the specular reflection coming from a car windshield. Another method of white balancing adjusts the image until the sum of all the areas in the image adds up to a neutral gray. Both of these methods are typically applied to the entire scene.
Applying a white balancing algorithm to the entire scene can be a problem when a strobe or flash is used in capturing the image of a scene. When a strobe, or auxiliary illuminating device, is used to enhance the illumination of the scene, typically the strobe will not have the same color temperature as the ambient light in the scene. When a strobe is used, nearby objects are more strongly illuminated by the strobe than objects that are further away. Objects that have higher reflectivity may also appear to be more strongly illuminated by the strobe. The power or intensity of the strobe is typically angle dependent. This means that the strobe illuminates the center of the scene more strongly than the edges of the scene. This causes the total illumination color of each object in a scene to be dependent on the distance between the camera and the object, the angle between the object and the center of the scene and the difference in the color temperature of the ambient light and the color temperature of the strobe. This makes it difficult to correct the scene for the variation in the color temperature as a result of strobe illumination of the scene. If the color temperature of the strobe or flash could be adjusted to match the color temperature of the ambient light, then the entire scene could be corrected or white balanced. Therefore, there is a need for a system that can adjust the color temperature of the auxiliary illuminating device to match that of the scene.
An auxiliary illuminating device is disclosed that has an adjustable color temperature. The color temperature is adjusted by varying the amount of light from a broad-band light source that is transmitted through a number of color filters. The color filters could be an array of red, green, and blue filters.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.